Field of the Invention
The present invention relates to an image reading apparatus in the field of an extensive image forming apparatus such as a copying machine, a facsimile apparatus, a printer, an image scanner, or a complex machine and a reflecting mirror unit used in the image reading apparatus.
Description of the Related Art
A conventional image reading apparatus which moves a scanning unit having a reflecting mirror formed thereon with respect to a focusing lens and a line sensor fixed at predetermined positions and exposes and scans an original surface to read an image is proposed by the present applicants (for example, see Japanese Patent Application Laid-Open No. 3-113961).
In recent years, on request of further miniaturization and simplification of an image reading apparatus, an image reading apparatus which constitutes a scanning unit which holds optical systems such as a reflecting mirror, a focusing element, a line sensor, and a light source together with each other as a compact unit and moves the scanning unit is practically used. As a device concerning to this, an image reading apparatus which constitutes an off-axial focusing unit to realize miniaturization is proposed (for example, see Japanese Patent Application Laid-Open No. 2004-126448). The off-axial focusing unit uses a plurality of mirrors as focusing elements, and the focusing mirrors are curved as off-axial reflecting surfaces having a direction opposing an incident direction of a reference axis beam to enable focusing.
However, the following several problems to be solved in common with conventional image reading apparatuses including the apparatuses disclosed in the Patent Documents are present.
As one of the problems, as in the image reading apparatus described in Japanese Patent Application Laid-Open No. 3-113961, in a structure for moving a scanning unit having a reflecting mirror, when a field angle is reduced to increase the resolution of a fixed focusing lens, the apparatus disadvantageously increases in size because an optical path length must be extended. From the opposite point of view, since a field angle must be increased to achieve miniaturization of the apparatus, the resolution at an end portion decreases, so that an amount of light decreases. In this manner, a high resolution of a lens and miniaturization of the apparatus are difficult to be compatibly satisfied.
Another problem, as in an image reading apparatus disclosed in Japanese Patent Application Laid-Open No. 2004-126448, a focusing mirror having an off-axial reflecting curved surface generally is made of a resin metal-molded and is not sufficient in rigidity or strength. For this reason, in order to fix a focusing mirror molded while keeping a mirror thickness constant by partially compressing the mirror or causing the mirror to adhere, stress acting on a pressing member or an adhering portion affects a mirror surface to deform the off-axial reflecting surface. As a result, focusing performance deteriorates.